Only my opinion, which doesn't amount for much with respect to vintage, but I'm inclined to agree with you Simon. I've been watching vinty auctions for at least a year now, all sorts including carded. I have never seen an ROTJ card with a coin.

The coin looks like a picture pasted on the card in from what I can see. Looks too flat sitting on the card & theres no clear plastic bubble to protect it. I took a quick shot of my ROTJ Teebo with a Imp. Dignitary coin on it & you can see that it has depth sitting on the card & it is more clear.

I've not heard of anything like this, either. I would agree and say that I also think it's just a case of somebody gluing a POTF coin onto a ROTJ card. I'm pretty sure it's not factory made simply because the POTF coins started with the POTF line, which came after the ROTJ line. So, the coins, in theory, were not even being produced when this card was made. Also, what cardback is it? It's not a 48-back, since that would have the Nien Nunb offer on the front, so it's a later version of the ROTJ series (65, 77 or 79). Basically, looks like a kitschy custom. Kinda neat to have, though. I wonder how the coin is attached?